how to pour a concrete slab when restoring a wooshed/barn so that the sills are supported

I have a wood shed attached to my 1820's farmhouse in Vermont that is in need of some TLC. It has great hand hewn beams and roof rafters seemingly straight from tree limbs with some bark still on them. Problem is the slate roof is bearing down on it and the foundation was 'farmer style' made of rock and slate. The sills are starting to twist a bit.

What is the best way to prepare the sills so that a concrete slab can be poured underneath to properly support the walls and begin the process of restoration. Clearly, putting in a metal roof and replacing the slate will take a big load off. But the foundation comes first I would have thought. I noticed on this site a video of a barn restoration and they used fiber glass in the concrete to reduce the structural rigidity. Makes sense. Anyone know someone good at this type of foundation restoration work?